504 MAMMALIA. 



We thus get the 5-cusped and 6-cusped teeth, which are usually 

 in the molar series. Finally the teeth may have more than six 

 cusps, in which case they are multicuspidate. When there is a 

 well marked talon the anterior part of the tooth has t5^pically, 

 though not always, three cusps arranged in the triangular 

 manner. 



Professors Cope and Osbom * have endeavoured to show that all cusped 

 mammalian grinding teeth may be referred to the tritubercular tooth, 

 usually with the addition of a cusp-carrying talon (tubercular-sectorial). 

 The cusps in tliis supposed primitive tooth and on its talon have been 

 named, and their homologies with the cusps of different mammalian teeth 

 determined. In this manner a theory of very great complexity, called 

 the tritubercular theory or simply trituberculism has been elaborated. 

 Without offering any opinion as to the vaUdity of this theory, we have 

 decided not to make use of it in this work, because the facts of tooth 

 structure in the different orders can be made sufficiently clear without it, 

 and because it does not appear to us to render that assistance in reducing 

 the facts to order which would justify us, at present, to ask the student 

 to make the considerable effort needed to master its complexities. 



Such grinding teeth in which the crown is elevated into blunt 

 or pointed cusps are called bunodont teeth. It frequently hap- 

 pens however that the cusps are spread out in one direction at 

 the base and connected by ridges. These ridges commonly run 

 transversely across the tooth, and when they are well developed 

 the crown of the tooth appears to be traversed by ridges of 

 enamel ; such teeth are called lophodont. In some cases the cusps 

 or tubercles are flattened at the base and spread out i'' a cres- 

 centic manner (as in Ruminantia). The crowns of such teeth 

 appear to be traversed by crescentic ridges of enamel arranged 

 of course partly in a longitudinal and partly in a transverse 

 direction. Such teeth are called selenodont. Lophodont and 

 selenodont teeth are found in herbivorous animals in which the 

 food, often dry and hard, requires much mastication. They are 

 thus subjected to considerable wear and the projecting enamel 

 on the crown becomes worn down. In this way the tops of the 

 cusps and ridges become worn off and the dentine exposed, and 

 the crowns of the teeth appear to be traversed by laminae of 

 enamel containing between them exposed dentine. It is clear 

 that when the crown is short, a tooth exposed to such hard wear 

 must soon be worn down to the neck. To obviate this, it fre- 

 quently happens that the crown, with its tubercles and cusps, 



1- Trituberculy, Amer. Nat., 1897, p. 993. 



